Amyloid-? (A?) peptide accumulation and aggregation in the brain lead to synaptic dysfunction and neurotoxicity. Increasing evidence indicates that impaired A? clearance is an early and central pathogenic event in Alzheimer's disease (AD). Our long-term goal is to understand the molecular and cellular pathways by which A? is either cleared from the brain or accumulated to form toxic aggregates. A? clearance and aggregation are regulated by A? chaperones and cell surface receptors. Apolipoprotein E (apoE) is a major A? chaperone and cholesterol carrier in the brain. Of the three apoE isoforms in humans (E2, E3, E4), the ?4 allele of the APOE gene is the strongest genetic risk factor for AD. Mounting evidence indicates that both apoE and apoE receptors, members of the low-density lipoprotein receptor (LDLR) family, play important roles in A? clearance, aggregation and toxicity. During our previous funding period, we have delineated the specific roles of a major apoE receptor, the LDLR-related protein 1 (LRP1), in neuronal A? uptake and trafficking. We found that neuronal LRP1 functions together with cell surface heparan sulfate proteoglycan (HSPG) to mediate A? uptake, trafficking and eventual delivery to lysosomes. Conditional knockout of Lrp1 in neurons in adult mice leads to increase A? accumulation and amyloid plaque deposition. Interestingly, while apoE3 promotes A? degradation, apoE4 facilitates A? accumulation and aggregation in the lysosomes. Finally, we found that the apoE receptor LRP1 and HSPG also play important roles in A? metabolism by astrocytes. Based on these observations, our overall hypothesis is that apoE and apoE receptor LRP1 and HSPG play critical roles in A? uptake and trafficking to lysosomes by neurons and astrocytes, and that the efficiency of A? clearance is differentially regulated by apoE isoforms. Thus, our overall goal for this competitive renewal application is to use both in vitro and in vivo models to systematically dissect how apoE isoforms and apoE receptors regulate A? clearance, aggregation and toxicity. We propose three specific aims to test our hypothesis. In Aim 1, we will use primary cultured neurons and astrocytes to analyze the specific roles of apoE isoforms and apoE receptors in A? endocytic trafficking to lysosomes, its degradation, aggregation and toxicity. In Aim 2, we will use in vivo microdialysis technique to assess the roles of apoE isoforms and apoE receptors in brain A? clearance. In Aim 3, we will use conditional knockout mouse models to examine the roles of apoE receptors in A? aggregation and pathology. Together, our proposed studies will likely provide insight into the mechanisms underlying A? clearance and aggregation pathways in the brain and uncover how apoE isoforms and apoE receptors regulate these events in AD.